Classification and structure Influenza viruses are spherical or pleomorphic, single-stranded, negative-sense RNA viruses belonging to the family Orthomyxoviridae. Influenza A and B viruses contain eight separate ribonucleoprotein (RNP) segments, while influenza C virus contains seven, each of which encodes 1 or 2 proteins. The internal antigens (M1 and NP proteins) are the type-specific antigens used to determine if a particular virus is A, B or C, while the external hemagglutinin (HA) and neuraminidase (NA) are the subtype- and strain-specific antigens.